Heretofore, in the Si IC process, etc. it is known that heat treatment of about 30 mm at 450C..degree. in a H.sub.2 atmosphere is useful as a measure for reducing damage (damage produced by irradiation with soft X-ray or ultraviolet ray, shock by charged particles such as electrons, etc.) in an MOS (Metal Oxide Semiconductor) by a plasma process.
However, in an element (correlator, convolver, etc.), in which a piezoelectric film such as ZnO or AlN is deposited on an MOS structure by the sputtering method and interaction of the potential of surface acoustic wave (SAW) with capacitance-voltage (C-V) characteristics, etc. of the MOS structure, since peeling of the piezoelectric film or cracks are produced by heat treatment at about 450C..degree., this method for reducing damage by heat treatment cannot be applied thereto.
Taking this point described above into account, when a piezoelectric film is formed by the sputtering method, a measure for reducing damage in the MOS as far as possible should be taken.
Taking a diode type RF sputtering apparatus as an example, it is known that means for setting an MOS substrate on a substrate holder has serious influences on damage in the MOS.
There are three measures, e.g. (a) the substrate holder is grounded; (b) the substrate holder is set at a floating potential; and (c) the substrate holder is set at a floating potential and in addition, an insulating plate is inserted between the substrate and the substrate holder. When these measures are compared with each other, the damage (voltage shift in the C-V characteristics of the MOS or increase in interfacial levels) in the MOS is reduced in the order of (a).fwdarw.(b).fwdarw.(c). That is, the damage is reduced more strongly, when the difference between the plasma potential and the potential of the substrate is as small as possible, in other words, when the potential gradient in the sheath in the neighborhood of the substrate is smaller. It seems that it can be thought that this is due to the fact that the strength of the shock by charged particles (ion or electrons) dominates the damage in the MOS in this case, although irradiation with soft X-ray or ultraviolet ray gives some damage.
However, for the measure (c) described above, it is necessary to optimize a parameter representing the thickness of the insulating plate (in the case where an insulating plate made of a predetermined material) inserted between the substrate and the substrate holder in addition to various parameters for fabrication such as gas pressure, RF power, and in the case of reactive sputtering, flow rate ration of sputter gas to reactive gas, etc.
Concretely speaking, if the thickness of the insulating plate is too great, the other parameters for fabrication being fixed, although the damage in the MOS is reduced, injection energy of the charged particles decreases, adhesive force is lowered, the stress in the film becomes tensile, and cracks are easily produced. On the contrary, if the insulating plate is too thin, the effect of reducing damage in the MOS is not satisfactory.
Consequently the thickness of the insulating film described above should be determined by trial and error for every fabrication condition and a very complicated procedure is required therefor.